The present invention relates to apparatus used in conducting endothermic reactions, and particularly to furnaces for reforming light hydrocarbons, especially mixtures of steam and/or carbon dioxide and light hydrocarbons.
Hydrogen for use in ammonia synthesis, methanol synthesis or hydrocracking plants, is frequently produced in a process using the following endothermic reforming reaction: ##STR1## The hydrocarbon can be methane, which is shown in the above reaction, or other light hydrocarbons, or mixtures of these such as natural gas. The same reactions may also be employed where the desired product is CO or both CO and H.sub.2. Another endothermic reaction is the conversion (pyrolysis) of ethane to ethylene which also produces the by-product hydrogen (H.sub.2), and this reaction requires heat only, usually about 950.degree. C. and 3 atmospheres pressure, but needs no catalyst. The catalyst over which the methane reaction is carried out is, for example, nickel on an inert support, such as alumina, and is contained usually in vertical tubes which are supported in a furnace frequently called a "reforming furnace". Reforming furnaces are disclosed or schematically shown in the process flow diagrams of U.S. Pat. Nos. 3,132,010; 3,264,066; and 3,382,044.
The tubes in the usual reforming furnaces extend vertically with reactants being fed by a manifold to one end of the set of tubes and product-rich gases being withdrawn from the other end of the tubes. Because the reforming reaction occurs at a high temperature and is endothermic, heat must be supplied to the tubes to heat the reactants. The reaction is carried out in the tubes at a high temperature of about 1500.degree. F. and at a pressure of about 150 to 450 psig.
Various efforts have been made to improve the performance of the reforming furnace or to improve the structure of the furnace to facilitate the maintenance and operation of the furnace. For example, U.S. Pat. No. 4,161,510 to A. J. Edridge teaches the use of a ceramic paint coating on the exposed surface of the reforming tubes in which the reaction is conducted, the paint being reflective to reduce fissure creep of the tubes. U.S. Pat. No. 4,714,593 to A. Naito et al. teaches a reforming apparatus which uses a catalyzed heat generating apparatus to achieve miniaturization of the furnace. U.S. Pat. No. 4,810,472 to S. P. Andrews et al. teaches a reforming apparatus which employs closed ended, double pass, externally heated, metal reformer tubes, also known as bayonet-tubes with controlled heat exchange between the product gas stream and the reactant stream across the wall of the tube.
Apparatus is also known which employs ceramic tubes for carrying out gas reactions in those ceramic tubes. For example, U.S. Pat. No. 2,987,382 to F. Endter et al. discloses such a ceramic tube furnace. Further, U.S. Pat. No. 2,018,619 to F. Winkler et al. teaches tubes made from material containing elementary silicon which may be embedded in other gas-tight tubes and which prevents carbon formation in carrying out pyrogenic conversion of hydrocarbons.
Known reforming furnaces are large and expensive, when measured as per unit of capacity, have tube lives shorter than would be desired, are prone to carbon deposition ("coking"), and must be frequently operated at temperatures below the ideally preferred values (for most efficient reactivity) due to the creep and corrosion behavior of their reforming tubes.
Therefore, it is an object of the present invention to provide an improved apparatus for conducting endothermic reactions, such as reforming light-hydrocarbons, and which will provide improved conversions, energy efficiency, and maintenance.